1. Field of the Invention
The present invention relates to an analog-to-digital (referred to as A/D) conversion method and device which serves to convert an analogue value into a digital value.
2. Description of the Prior Art
A conventional analog-to-digital conversion device is schematically shown in FIG. 2. An analogue input signal 1 is applied to a common input terminal of a first comparator chain 2. The other input terminals of the first comparator chain 2 are connected to a first reference voltage generating means 3. The reference voltages generated in the means 3 are respectively applied to the other input terminals of chain 2. The outputs of the first comparator chain are logically converted by a first logic circuit 4 for providing upper-digits of the converted value. Additionally, the outputs of the first comparator chain are sent to a digital-to-analogue conversion section 5 in which they are converted into an analogue signal. The resulting analogue signal is sent to a subtracter 6 in which a potential difference between the analogue signal is subtracted by the origianl analogue input signal. Then, the subtracted signal is sent to common input terminals of a second comparator chain 8. The other input terminals of the second comparator chain 8 are connected to a second reference voltage generating means 7 so that the reference voltages generated in the means 7 are respectively applied to those other input terminals. The comparing outputs of the second comparator chain are logically converted by a second logic circuit 9 for providing lower-digits of the converted value.
The conventional A/D conversion device, however, is likely to generate a large conversion error due to instability of the gain of the subtracter 6, insufficient precision of reference voltages generated in the second reference voltage generating means 7, or variation of the offset voltages in the first comparator chain 2. It has been thus difficult to carry out the conversion quite accurately. The present invention is designed to overcome this difficulty. It is, therefore, an object of the invention to provide an A/D conversion method and device which is capable of performing high-accuracy A/D conversion as well as a comparator circuit which implements the A/D conversion device.